A light detecting sensor has a cathode discharging photoelectrons by the incidence of light, and an anode collecting the photoelectrons, and in general, a transmissive light detecting sensor and a reflective light detecting sensor exist. In the transmissive light detecting sensor, the cathode is provided on a light incident side to the anode, and in the reflective light detecting sensor, the anode is provided on a light incident side to the cathode.